1. Field of the Invention
The present invention generally relates to a method and apparatus for measuring a material in a slurry during a polish process, and more particularly to a method and apparatus for accurately monitoring polish processes by measuring a removed metal concentration in the slurry.
2. Description of the Related Art
Copper and Tantalum liner polish operations typical of current copper back end of line (BEOL) (and aluminum BEOL in some cases) interconnect layers involve the bulk removal of metal by chemical mechanical polishing (CMP) on processed semiconductor wafers.
In operation, there is some level of interest to which the designer wishes to make connections. To do so, an oxide is deposited over the level, lines and holes (vias) are made in the oxide, patterned therein and extending down to the level of interest (e.g., the layer to which the connections are to be made). Then, a liner (e.g., formed of tungsten, tantalum, etc.) is formed over the wafer surface and in each of the holes, and then a copper layer is electroplated over the structure. Thereafter, the copper layer is polished back until the liner is exposed everywhere except where there are trenches or holes. Thus, all that is left is the interconnect wires, and the bulk copper is removed.
A condition arises in that, as the final stage of polishing is reached, the copper concentration in the slurry drops, as the relatively thin liner (which has a possibly different selectivity to the polish) is reached, and thus the liner becomes present in the slurry (e.g., because the liner is now being ground). The liner components in the slurry typically exist in fairly low concentrations (e.g., about 30 ppm to about 50 ppm). Thus, detection is somewhat difficult. It is desirable at this point to stop the copper polish before a large amount of liner has been removed. Prior to the invention, no method has existed which could precisely stop the polishing process to avoid polishing the liner.
Thus, the correct performance of these process operations depend on each polishing step being stopped precisely as the material of each layer is depleted.
Prior to the present invention, no conventional method has been developed which can achieve such precise stopping.
Further, it is noted that thermal endpoint methods exist which take advantage of the evolved heat due to friction. As the liner is reached, the friction between the polish pad and wafer in the presence of slurry changes. This results in a sensible change in temperature in some polish operations. Due to the thermal masses involved and the thermal time constant of the system, it takes a certain amount of time to sense the change (typically several seconds or more). Thus, the thermal endpoint method does not work for all processes.